Highly cross-linked polymers have been studied widely as matrices for composites, foamed structures, structural adhesives, insulators for electronic packaging, etc. The densely cross-linked structures are the basis of superior mechanical properties such as high modulus, high fracture strength, and solvent resistance. However, these materials are irreversibly damaged by high stresses due to the formation and propagation of cracks. Polymerization stress is originated from polymerization shrinkage in combination with the limited chain mobility. Which eventually leads to contraction stress concentration and gradually such a trapped stress would released and caused microscopically the damage in certain weak zone like interfacial areas. Macroscopically it was reflected as debonding, cracking, et al. Similarly, The origin of contraction stress in current adhesive restorations is also attributed to the restrained shrinkage while a resin composite is curing, which is also highly dependent on the configuration of the restoration. Furthermore, non-homogeneous deformations during functional loading can damage the interface as well as the coherence of the material. Various approaches have been exploring by limiting the overall stress generation either from the restorative materials, or by minimizing a direct stress concentration at the restored interface. It included, for example, new resin, new resin chemistry, new filler, new curing process, new bonding agent, and even new procedure.
There have been tremendous attention paid on new resin matrix development that could offer low polymerization shrinkage and shrinkage stress. For example, various structure and geometry derivatives of (meth)acrylate-based resin systems; non-(meth)acrylates resin systems, non-radical-based resin system. In addition, for light curable, low shrink dental composites, not only new resin systems and new photoinitiators, new filler and filter's surface modification have also been extensively explored, such as filler with various particle size and size distribution, from nanometer to micrometer, different shape, irregular as milled or spherical as-made. It can also be different in composition like inorganic, organic, hybrid. Although an incremental improvement has been achieved with each approach and/or their mutual contribution, polymerization stress is still the biggest challenge in cured network systems.
According to one aspect of the invention, a new kind of resin composition is provided. However, unlike conventional resin system, a new concept is involved in designing such a new resin composition, which would render the polymerization stress in post-gel stage to a subsequent, selective network cleavage in order to have the stress partially released. As mentioned above, all of previous arts towards low shrink and low stress are based on the limitation on the shrink and stress formation in general. However, the shrinkage and stress development in cured network system should have two different stages: a pre-gel phase and a post-gel phase. Actually, most efforts of current arts are focussed on the pre-gel stage and some of them were proved to be effective. Unfortunately, these approaches become ineffective in terms to control the stress development in post-gel stage, where the shrinkage is not as much as in the pre-gel stage but the stress turns to much more sensitive to any polymerization extend. It is the immobility nature of the increasing cross-link density within the curing system that leads to the increasing stress concentration within the curing system, period. Even worse, the problem does not stop here and the trapped stress would eventually get relief from slow relaxation, which can create additional damage on a restored system. Therefore, our approach is based on such a concept that in the post-gel stage if some of “closed net” of any cross-linked system can be selectively broken to promote an extended stress relief period, the total stress concentration would be substantially reduced. To fulfil such a task, a photopolymerizable and photocleavable resin is proposed and a general molecular constitution is designed. It was expected that such a resin monomer can be polymerized like any other resin monomer but its mainframe is able to be triggered to break upon additional light source such as near UV is blended. This is a typical photocleavable process, but it is its capability to be photopolymerized and embedded into a cross-linked system make it unique. In addition, it also makes possible to avoid regenerating any leachable species through such secondary breakage.
Photocleavage is nothing new in solid synthesis of peptides, from which new peptides was directed on certain template in designed sequence, then it was cleaved from its template via a subsequent light exposure. There is no chemical contamination with such a process. On the other hand, photoacid and photobase could be viewed as extended applications for photocleavage. Acidic or basic component is temporally latent to avoid any unwanted interaction with others in the system and they can be released on demand such as light exposure to trigger the regeneration of the acid or base, which then act as normal acidic or basic catalyst for next step reactions. Recently, thermally removable or photo-chemically reversible materials are developed in order to make polymer or polymeric network depolymerizable or degradable for applications such as easily removing of fill-in polymer in MEMS, thermally labile adhesives, thermaspray coatings and removable encapsulation et al. Most recently, photocleavable dentrimers are explored in order to improve the efficiency for drug delivery. Based on our knowledge, there is no prior art involved photocleavable segment in cured network for contract stress control. However, all of those related arts could be used as a practical base to justify this investigation.
Dental composite is formulated by using organic or hybrid resin matrix, inorganic or hybrid fillers, and some other ingredients such as initiator, stabilizer, pigments et al so as to provide with the necessary esthetic, physical and mechanical property for tooth restoration. It is well known that polymerization shrinkage from cured dental composite is one of dental clinicians' main concerns when placing direct, posterior, resin-based composite restorations. Although there are evolving improvements associated with resin-based composite materials, dental adhesives, filling techniques and light curing have improved their predictability, the shrinkage problems remain. In fact, it is the stress associated to polymerization shrinkage that threaten marginal integrity and lead to marginal gap formation and microleakage, which may contribute to marginal staining, post-operative sensitivity, secondary caries, and pulpal pathology.
A common approach to redue the polymerization shrinkage of dental composite is to increase the filler loading, especially for posterior restoration. However, the higher viscosity of these highly filled composites may not adapt as well to cavity preparations. 1-2 It has been demonstrated that to initially place a flowable composites which, with less filler content, have greater flexibility, could reduce microleakage than direct application of microhybrid and packable composite restorations, 3-4 but this benefit may be offset by the increasing polymerization shrinkage for the flowable composite itself.5 Therefore, it is also highly desirable to develop low shrinkage, especially low curing stress flowable composite, in order to really reduce microleakage as mentioned above.
The challenge in developing any dental composite is to balance the overall performance, including esthetic appearance, handling character as well, in addition to low curing stress and necessary mechanical strength. Unfortunately, superior mechanical strength usually is a result of increasing cross-linking density, from which an unwanted polymerization shrinkage and shrinkage stress always accompanied. There is increasing effort to develop new resin systems in the attempt to minimize such a shrinkage and stress accordingly. For example, reducing the polymerizable proups in the resin matrix by designing resin monomer with different size and shape indeed work well to some extent in this regard. However, it is usually resulted in decreasing mechanical strength and losing certain handling characteristic because of the limited molecular chain mobility and the limited polymerization conversion. In addition the shrinkage can also be reduced by using special filters which allow an increase in filler loading without compromising too much in handling property. Even so, the curing stress from most of flowable composites remains substantially high. Obviously, it is highly desirable to develop flowable dental composition with low curing stress.